Fellaini has bounced back from an early struggle to become one of Manchester United's key assets under van Gaal.Getty Images

Ashley Young has lauded the impact of teammate Marouane Fellaini after Manchester United's impressive derby victory over Manchester City.

Both players were on excellent form on a hugely satisfying afternoon for home supporters at Old Trafford, as Louis van Gaal's side successfully avenged the 1-0 defeat suffered to their closest rivals in November after rallying in fine fashion to turnaround an early deficit.

Fellaini, who was heavily criticised by many in the stands and indeed in the press during a difficult first season at the club following his £27.5m move from Everton, was named man of the match for yet another excellent contribution and Young believes the Belgian international is simply unplayable on this sort of form.

"Felli has been brilliant. Everyone will say the same. Since he's come into the team, he's been fantastic. I've said before that very few people can handle him. On his day, he's unplayable. I don't think Yaya Toure wanted to be tracking back, he wanted to be at the other end of the pitch but he had that job to do on Felli," Young told MUTV.

"When Felli is in the box and you can find him, you've definitely got a chance of scoring. I saw him at the back post and hoped he'd get on the end of it and score and, thankfully, he managed to do that."

While Fellaini once again excelled at putting opposition defenders under severe pressure with his size and ability to roam menacingly throughout the opposition half, Young also attracted significant praise for his latest impressive performance.

After notching the opening goal courtesy of a cross from Ander Herrera, the 29-year-old also provided the assist for Fellaini to take United into a lead which they would not relinquish with the evergreen Michael Carrick dominating in midfield before being forced off with a calf problem.

His free-kick also led to Chris Smalling adding a fourth for the hosts late on after Eliaquim Mangala was guilty of woefully inadequate marking.

As one of several underperforming players who was expected to be marginalised further with van Gaal looking to re-shape the squad following a torrid campaign under the management of David Moyes, Young's renaissance has gradually led to Angel di Maria, the most expensive signing in British football history at £59.7m, being confined to the substitutes' bench alongside the likes of Radamel Falcao.

Such a work ethic has been integral to a run of six consecutive victories that has led to United to opening up a four-point gap over City in third and all but secured their place in next year's Champions League.

A difficult trip to leaders Chelsea is next on the horizon for a resurgent United, but for now Young and co appear eager to soak up the adulation that inevitably follows such a morale-boosting victory.

"A brilliant afternoon! A great team performance and you can't beat Old Trafford for atmosphere when it's that noisy, our fans were immense! Hope everyone has a great evening #MUFC#ManchesterIsRED," he posted on Instagram after the match.